1. Field of the Invention
This invention relates to a method of manufacturing a spiral electrode assembly. More particularly, the present invention relates to a method of manufacturing a spiral electrode assembly which comprises a positive electrode member and a negative electrode member wound into a spiral shape together with a separator interposed therebetween and which is used in, for example, a cylindrical nonaqueous electrolyte type cell or a cylindrical alkaline storage battery.
2. Description of the Related Art
A method used hitherto for the manufacture of a spiral electrode assembly of this kind has comprised the steps of inserting one of a positive and a negative electrode member covered with a separator into a slot formed in a spool, winding the electrode member around the spool, and, while winding the electrode member, inserting the other electrode member to wind the positive and negative electrode members separated by the separator around the spool.
However, according to this prior art method, the first-mentioned electrode member is bent at an angle of about 90.degree. at the area where it protrudes from the slot of the spool to be wound around the spool. As a result, this electrode member tends to be torn at that area when it is made of a soft metal material having a low mechanical strength such as lithium, sodium or aluminum. In another case, the electrode member is broken at the bent portion, and this broken portion of the electrode member or the active material dropped off the electrode member penetrates the separator to make contact with the adjacent electrode member of opposite polarity thereby giving rise to an objectionable internal short-circuit.
With a view to obviate such a trouble, a method has been proposed in which a protective sheet is attached to each of the leading and trailing ends of the turns of an electrode member, as disclosed in U.S. Pat. No. 3,298,871. However, the proposed method has not been completely successful in that the protective sheets tend to be stripped off during winding the electrode member and, also, the electrode member tends to be broken at its bent portion, resulting in break-through of the corresponding portion of the protective sheet.
A method which improves the method disclosed in the aforementioned U.S. patent has been proposed in, for example, Japanese Patent Publication No. 47-50417 (1972). According to the method proposed in the Japanese Patent Publication, a sheet having a predetermined length, which is in the form of a film or a woven cloth of a synthetic resin, is attached at one end thereof to the leading end of an electrode member covered with a separator, and this sheet is inserted at the other end thereof into a slot of a spool and is then wound around the spool. The proposed method is advantageous in that the electrode member can be wound around the spool without being bent. However, since the sheet is attached to the leading end only of the electrode member, the sheet tends to be also detached or peeled during the winding operation. Also, a considerably large winding force cannot be applied to the sheet at the beginning of the electrode winding operation. Therefore, the electrode group completed by the winding operation is not sufficiently tight.
Further, JP-A-No. 50-55843 discloses a method comprising extending a separator beyond the leading end of an electrode member, reinforcing the separator extension by compacting it by heat fusion, inserting this reinforced separator extension into a slot of a spool, and winding the separator extension to wind, the electrode member around the spool. According to the proposed method, however, the separator extending from the leading end of the electrode member includes a portion of a large length where the separator function is lost as a result of the heat fusion, and the cell activity is lost in that portion resulting in an undesirable reduction of the cell capacity.